Abstract
PurposeTo quantify the differences in dosimetry as a function of ipsilateral lung density and treatment delivery parameters for stereotactic, single dose of volumetric modulated arc therapy (VMAT) lung stereotactic body radiation therapy (SBRT) delivered with 6X flattening filter free (6X‐FFF) beams compared to traditional flattened 6X (6X‐FF) beams.Materials/methodsThirteen consecutive early stage I–II non‐small‐cell‐lung cancer (NSCLC) patients were treated with highly conformal noncoplanar VMAT SBRT plans (3–6 partial arcs) using 6X‐FFF beam and advanced Acuros‐based dose calculations to a prescription dose of 30 Gy in one fraction to the tumor margin. These clinical cases included relatively smaller tumor (island tumors) sizes (2.0–4.2 cm diameters) and varying average ipsilateral lung densities between 0.14 g/cc and 0.34 g/cc. Treatment plans were reoptimized with 6X‐FF beams for identical beam/arc geometries and planning objectives. For same target coverage, the organs‐at‐risk (OAR) dose metrics as a function of ipsilateral lung density were compared between 6X‐FFF and 6X‐FF plans. Moreover, monitor units (MU), beam modulation factor (MF) and beam‐on time (BOT) were evaluated.ResultsBoth plans met the RTOG‐0915 protocol compliance. The ipsilateral lung density and the tumor location heavily influenced the treatment plans with 6X‐FFF and 6X‐FF beams, showing differences up to 12% for the gradient indices. For similar target coverage, 6X‐FFF beams showed better target conformity, lower intermediate dose‐spillage, and lower dose to the OAR. Additionally, BOT was reduced by a factor of 2.3 with 6X‐FFF beams compared to 6X‐FF beams.ConclusionWhile prescribing dose to the tumor periphery, 6X‐FFF VMAT plans for stereotactic single‐dose lung SBRT provided similar target coverage with better dose conformity, superior intermediate dose‐spillage (improved dose coverage at tumor interface), and improved OAR sparing compared to traditional 6X‐FF beams and significantly reduced treatment time. The ipsilateral lung density and tumor location considerably affected dose distributions requiring special attention for clinical SBRT plan optimization on a per‐patient basis. Clinical follow up of these patients for tumor local‐control rate and treatment‐related toxicities is in progress.
Highlights
Due to the recent technological advances in lung stereotactic body radiation therapy (SBRT) treatments and reported comparable tumor local‐control rates,1–7single‐dose lung SBRT has become a viable treatment option for peripherally located lung lesions for medically inoperable early‐stage nonsmall‐cell lung cancer (NSCLC) patients.[8,9,10] there has been growing interest in the clinical use of flattening filter‐free (FFF) beams to deliver lung SBRT treatment.[11,12,13,14,15] FFF‐beams have much higher dose rates compared to flattened beams and reduce beam on time significantly
The 6X flattening filter free (6X‐FFF) plans had the advantage of providing less intermediate dose‐spillage compared to traditional 6X‐FF plans
Mean dose to the Internal target volume (ITV) was similar between the plans, the maximum and minimum doses to the ITV were significantly higher for 6X‐FF plans compared to 6X‐FFF plans
Summary
Due to the recent technological advances in lung stereotactic body radiation therapy (SBRT) treatments and reported comparable tumor local‐control rates,1–7single‐dose lung SBRT has become a viable treatment option for peripherally located lung lesions for medically inoperable early‐stage nonsmall‐cell lung cancer (NSCLC) patients.[8,9,10] there has been growing interest in the clinical use of flattening filter‐free (FFF) beams to deliver lung SBRT treatment.[11,12,13,14,15] FFF‐beams have much higher dose rates compared to flattened beams and reduce beam on time significantly. Linac‐based intensity modulated radiation therapy (IMRT), helical TomoTherapy or optimized robotic CyberKnife treatments significantly prolong SBRT treatment time, comparatively.[18,19,20,21]
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